The cost of generating solar energy per unit Watt in a photovoltaic system is several times higher than generating energy from other sources, which include coal, oil, wind, biomass and nuclear energy. In order to reduce the cost of solar energy generation in photovoltaic systems, it is desirable to make efficient use of the most expensive part of the system, namely the photovoltaic cell (herein also referred to as the solar cell). Conventionally, this is done by using large light-focusing parabolic mirrors or large Fresnel lenses, with the photovoltaic cell positioned in the focal point of the parabolic mirror or Fresnel lens. These devices have several disadvantages, including high investment cost, high maintenance cost, unwieldy shapes, and the necessity of tracking the sun as it crosses the sky, requiring mechanical constructions to physically alter the orientation of the device, the mirrors, lenses and/or the photovoltaic cell (PV cell). Photovoltaic systems have thus been the subject of many recent investigations, in particular to provide cost-effective and efficient alternatives allowing concentration of incident light collected over a large area to a small area.
Luminescent solar concentrators (LSC) represent an alternative that has been the subject of investigations, predominantly because these systems are easy to produce at low cost and because these systems do not require tracking of the sun. Luminescent solar concentrators are known in the art and usually consist of a flat plate which acts as a waveguide, usually plastic, either filled with a single fluorescent dye, phosphor and/or quantum dot or a plurality of these materials in a uniform mixture, or with these luminophore(s) in a thin layer (<100 microns) on the top or bottom of the waveguide. The luminescent material absorbs light of specific wavelengths from the solar light incident upon it, and re-emits the light in all directions at longer wavelengths. A portion of this light is emitted outside the critical angle of a supporting waveguide, and is totally internally reflected and transported to the photovoltaic cell, typically positioned at one or more ends of the waveguide for conversion of the emitted light to electricity. In this way, luminescent solar concentrators concentrate sunlight to a particular spot or area, where the concentrated solar energy can then be converted by a PV solar cell. This not only increases efficiency, but also decreases cost, as luminescent solar concentrator panels can be made cheaply from plastics, while PV-cells need to be completely constructed from expensive materials such as purified silicon. The LSC has the advantage of combining less expensive materials with flexibility (especially when a plastic waveguide is used) without the need of a heat sink or a sun tracking system. At the moment, LSC-systems are not used commercially which is predominantly related to their relatively poor efficiency. This low overall efficiency originates from a high re-absorption of emitted light (limited Stokes Shift of the dye), from a poor efficiency of coupling light into the waveguide and from a poor efficiency in keeping the light within the waveguide.
An example of an alternative LSC system is described in European Patent EP 1 854 151 B1. EP 1 854 151 describes, amongst others, a laminate of a planar waveguide with a luminescent layer, arranged to receive incident sun light over a full face of the laminate, to excite the luminescent layer with the incident sun light, upon which the luminescent layer emits emission light into the waveguide, and to transport the emission light through the waveguide to an exit surface of the waveguide, where it is delivered to a PV cell. This may allow the use of small-sized PV cell(s), as the waveguide may have a small exit surface(s), while it may have a large light collection surface, as the waveguide may have a large area. The known laminate may be advantageous over alternative known systems as it provides a thin, potentially light-weight and potentially allows roll-to-roll manufacturing. However, the known laminate may have the disadvantage that the emission light may be reabsorbed by the luminescent layer when propagating through the waveguide, or be lost upon scattering by the luminescent layer. Although part of the reabsorbed light may be re-emitted by the luminescent layer, the associated losses may be significant, especially when the waveguide has a large area and is thin, and the propagating emission light is interacting multiple times with the luminescent layer. Another disadvantage may be that the relative area between waveguide and size of the PV cell may be limited, due to these propagation losses.
Further, luminescent solar concentrators are solar energy collectors that are well suited for integration into existing products such as buildings, sign-boards, and similar objects. However, in many of these applications, the appearance of the system is as important, or even more important, than its functionality. For an acceptable appearance, it is often desirable to have a luminescent solar concentrator that displays multiple colors, with the colors in specific regions and visible as separate colors.
Traditionally, additional colors are being applied to LSC's using non-luminescent dyes. The technology to do so is widely available and the patterning of the dye on the substrate is easily implemented using offset printing, flexoprinting, painting, or other techniques. However, in the case of an LSC, patterning of a non-luminescent dye on an LSC dramatically decreases the output of the solar energy collection.
There is thus a desire for an alternative thin and efficient solar energy system, and components therefore. In particular, there is a desire to provide a luminescent optical device as a light concentrator therefore in the form of a thin, light-weight structure, in particular in the form a sheet or a laminate of a plurality of layers. It is another desire to provide a potentially cheap and robust device. It is another desire to provide a device which may be economically manufactured in a variety of sizes, e.g. in a roll-to-roll process, and/or be suitable for mass-scale manufacturing.
It is a further object of this invention to provide a luminescent solar collector that provides the desired appearance for certain applications without negatively affecting the electrical output.
Hence, it is an aim of the invention to provide an alternative luminescent optical device, which preferably further at least partly obviates one or more of the above-described or other drawbacks associated with the prior art, and which further preferably fulfils one or more of the above indicated desires. In particular, the invention aims to provide a luminescent optical device with an increased efficiency with which emitted light is transported through the waveguide.
In another aspect, the invention aims to provide an alternative solar cell system, which preferably further at least partly obviates one or more of above-described drawbacks, and which further preferably fulfils one or more of the above indicated desires.